1. Field of the Invention
This invention relates to the removal of organic contaminants from aqueous solutions, and in particular to a process for the removal of organic compounds which are amenable to electrochemical coupling in an electrochemical cell.
There is presently great interest in methods for economically removing and disposing of organic toxins from waste water. For the purposes of this application, the term "Waste water" is used to refer not only to effluent flows from chemical processes, but also to contaminated surface and ground water. Some of the methods presently used involve adsorption onto active carbon followed by landfill, incineration or regeneration, and direct chemical treatment (e.g. hydrogen peroxide addition activated with ultraviolet light). There are also some proposed electrochemical treatment systems which involve direct oxidation or reduction of waste water to achieve destruction or detoxification of the toxic organics.
Active carbon is commonly used for adsorbing impurities from waste streams and from inlet streams in water treatment. The removal of impurities involves physical adsorption of the materials onto the surface of the active carbon. After the surface of such carbon will no longer adsorb organic (i.e. the carbon surface is at equilibrium with the waste at some, typically, sub-monolayer coverage) the carbon must be removed from service. At such time the carbon can be disposed of in landfill, by incineration, or some method of regeneration must be used. Regeneration commonly involves heating the carbon to high temperature to volatilize the adsorbed species and the resulting off gases are incinerated and/or scrubbed. Such thermal regeneration results in the loss of around 10 wt. % of the carbon material, implying that after c.a. 10 regenerations all of the original carbon has been consumed. Also, the active carbon must be removed from the adsorption unit and transported to a suitable thermal regeneration facility (where suitability refers to the degree of off gas scrubbing/treatment and is dependent on the nature of the adsorbed material). Thus, any method of increasing the amount of material which can be loaded on the carbon and thereby extending the time between regenerations would be useful.
2. Description of the Prior Art
Methods of direct treatment of waste water include chemical treatment (using oxidants such as bleach, peroxide or metal ions) and electrochemical treatment (anodically to mineralize organics either directly or using reactive intermediates, and cathodically to reduce the toxicity by dehalogenating, for example chlorinated organics). These direct treatment methods are limited by the low concentration the organic toxins usually found in waste water leading to low reaction rates and poor efficiencies (chemical efficiency for chemical oxidants or current efficiency for electrochemical treatments). Direct treatments are also limited because of the need to add to the waste water stream chemicals such as: reactants, catalysts or highly conductive (i.e. high concentration) electrolytes, and often the need to later recover or neutralize them.
Biological treatments tend to be slow, requiring a large hold up volume, and therefore a large equipment size. Such treatments, especially when the waste water contains fairly toxic compounds, tend to be temperamental and for dilute streams require the continuous addition of nutrient mixtures.
Condensation reactions of many compounds are well known in organic electrochemistry (1). These reactions typically involve the generation of radical intermediates, through electrochemical oxidation or reduction, these then react leading to the formation of higher molecular weight, less soluble compounds. Most fundamental work on these reactions has been done in non-aqueous environments (1) though some aqueous work has been carried out involving the use of electrochemical polymerization (from relatively high concentration solutions) for coatings production (2). Such condensation products have also been noted in electro-organic synthesis (3) and direct electrochemical waste water treatment (4), where the resulting loss of electrode activity due to the build-up of condensation products on the electrode presents a problem with some processes.
This collection of toxic compounds from a waste stream by condensation is fundamentally different from the commonly known approach of electrosorption. Electrosorption involves applying a potential to an electrode in order to make the adsorption of a compound more favourable and can involve the partial or complete transfer of an electron from the target compound to form a surface bound spieces. The electrode used can also be a high specific surface area carbon electrode such as packed beds of granulated active carbon (6) or active carbon cloth (7). Electrosorption, however, tends to be chemically reversible allowing adsorption and desorption cycle to be used to concentrate a target compound (see (8) for example). Also, while electrosorption can produce a perturbation of an adsorption isotherm or the formation of a surface bound species, in both cases it is limited to monolayer coverage.